Project Summary Regulatory T (Treg) cells play a dominant role in controlling immune responses by the suppression of other effector T cells to prevent excessive reactions to benefit the body. On the other hand, Tregs are recruited to and induced by tumor cells, which results in hampering protective anti-tumor immunity to cause body damage. However, the detailed mechanisms for Treg-mediated immune tolerance are poorly characterized. The E3 ligase component Fbw7 mediates the turnover of multiple proto-oncogenes that are broadly implicated in tumorigenesis such as the T cell acute lymphoma. However, whether Fbw7 signaling is crucial to Treg cell function remains unknown. We initiated this project by generating Foxp3+ Treg cell-specific deletion of Fbw7 in mice. Notably, Fbw7f/f Foxp3-YFPcre mice showed markedly reduced lung metastasis and we also found that an enhanced activation and cytokine production by conventional T cells. In addition, Fbw7 belongs to a family of WD-domain containing E3 ligases with ~26 members, and most of them have not been characterized so far. We hypothesize that the E3 ligase component Fbw7 and its family members play overlapping or redundant roles in immune tolerance by regulating the function of Treg cells. To test this hypothesis, we propose the following two Specific Aims: Specific Aim 1. To study the cellular and molecular mechanisms of Fbw7 in Treg cells, and Specific Aim 2. To study Fbw family members in Treg cells. This exploratory R21 application will allow us to study a novel function of the E3 ligase component Fbw7 and its family members in immune tolerance and anti-tumor immunity. The expected results will significantly advance our knowledge on the basic biology of the ubiquitin system in immune regulation, and on the translational aspect of harnessing Treg cells in immunotherapy.